357. Count Numbers with Unique Digits

class Solution {public:    int countNumbersWithUniqueDigits(int n) {        if(n==0)            return 1;        int ret=10;        int unique=9;        int available=9;    while(n>1&&available>0)        {            unique=unique*available;            ret+=unique;            available--;            n--;        }        return ret;    }};